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Icom IC-737 (IC 737 IC737) CW Filter Switching Modification
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The Icom 737 is a wonderful radio for CW operation, but I did not like
the filtering switching options offered in normal operation of the radio
when you have installed BOTH the 9 Mhz and 455 Khz optional CW filters.
The default option is to use the SSB filter for CW in the CW mode, and
then switch in BOTH the 9 Mhz and 455 Khz optional IF filters in the CW-N
mode. This provides too broad a filter for general tuning in CW, and
the CW-N mode (with the two filters in) is far too sharp for just
tuning around.
I felt that it would be better to allow the 455 Khz filter to be
functioning full time in the CW mode, and then switch in the 9 Mhz CW
filter in the CW-N mode. The following simple modification only requires a
single wire to added to the radio.
MODIFICATION
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Remove the top/bottom cover of the radio as per the manual.
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Position the radio facing you and locate the resistor R-323 which is
simply labeled "323" on the PC board just to the left of the opional
455 Khz CW filter. Solder a wire to the end of R-323 which is closest
to the front pannel.
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Locate J-3 just above and to the right of the 9 Mhz filter F-13.
Attach a wire to the connector or wire going to pin 7. Pin 7 is the
third pin from the right. (The pins are numbered 1-9 from left to right)
You may carefully tack solder a wire to the plug by removing it first.
I used a solid 28 gauge wire and just pushed it into the connector without
soldering.
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Replace the top and bottom covers.
I'm happy with the function of this modified switching arrangement.
The CW-N function is now the mode of last resort when going after the weak
ones, or when the QRM gets out of control, and tuning with just the 455 Khz
filter is a better than using the SSB filter.
Pin 7 of J3
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|ooooooooo|
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| F13 | |
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| CW FILTER | |
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| | ------------
| - | CW |
New wire >| R-323 | | | FILTER |
| - ------------
+--------|
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FRONT
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